Object location system and method using RFID

ABSTRACT

A system and method is provided for locating objects using RFID tags. The system and method uses an RFID reader and a distance calculator to efficiently and accurately determine the location of objects that include an RFID tag. The RFID reader transmits a plurality of signals to the RFID tag, with the plurality of signals having different fundamental frequencies. In response, the RFID tag backscatter modulates the plurality of transmitted signals to create a plurality of backscatter modulated signals. The RFID reader receives and demodulates the plurality of backscatter modulated signals. The distance calculator determines the phase of the plurality of backscatter modulated signals and determines a rate of change of the phase in the backscatter modulated signals with respect to the rate of change in the fundamental frequency of the transmitted signals and uses this information to calculate the distance to the RFID tag.

FIELD OF THE INVENTION

This invention generally relates to object location systems, and morespecifically relates to object location systems and radio frequencyidentification tags (RFIDs).

BACKGROUND OF THE INVENTION

In modern supply management systems there is a growing need for theability to locate and track a wide variety of objects. This can includethe locating and tracking of objects over wide areas, such as largefactories and distribution centers.

Unfortunately, previous attempts creating such a system have eitherrelied upon relatively expensive technologies such as GPS, or havefailed to provide the accuracy and reliability desirable for such asystem. For example, GPS based systems have relied upon complex batterypowered devices with high per-unit costs and limited battery life. Thecost and accuracy limitations of these previous methods have preventedtheir adoption in applications that needed to provide the ability totrack and locate large numbers of objects at a relatively low per-unitcost.

Accordingly, it is desirable to provide an improved method for locatingobjects. In addition, it is desirable to provide an improved system forlocating objects. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a system and method for locating objectsusing RFID tags. The system and method uses an RFID reader and adistance calculator to efficiently and accurately determine the locationof objects that include an RFID tag. The RFID reader transmits aplurality of signals to the RFID tag, with the plurality of signalshaving different fundamental frequencies. In response, the RFID tagbackscatter modulates the plurality of transmitted signals to create aplurality of backscatter modulated signals. The RFID reader receives anddemodulates the plurality of backscatter modulated signals. The distancecalculator determines the phase of the plurality of backscattermodulated signals and determines a rate of change of the phase in thebackscatter modulated signals with respect to the rate of change in thefundamental frequency of the transmitted signals and uses thisinformation to calculate the distance to the RFID tag.

In one specific embodiment, an array of RFID readers is used todetermine the object location. Again, each of the RFID readers transmitsa plurality of signals to the RFID tag, with the signals from each RFIDreader having different fundamental frequencies. In response, the RFIDtag backscatter modulates the plurality of transmitted signals to createa plurality of backscatter modulated signals. The array of RFID readersreceives and demodulates the plurality of backscatter modulated signals.The distance calculator determines the phase of the plurality ofbackscatter modulated signals and determines a rate of change of thephase in the backscatter modulated signals with respect to the rate ofchange in the fundamental frequency of the transmitted signals and usesthis information to calculate the distance between each RFID reader andthe RFID tag. Using the distances to the array of RFID readers and theknown location of the readers, an accurate location can be determinedusing trilateration techniques. Thus, the system and method is able toefficiently determine an accurate location for objects that include anRFID tag.

BRIEF DESCRIPTION OF DRAWINGS

The preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and:

FIG. 1 is a schematic view of an object location system in accordancewith an exemplary embodiment of the present invention;

FIG. 2 is a flow diagram of a method for locating an object inaccordance with an exemplary embodiment of the present invention;

FIG. 3 is a schematic diagram of an array of RFID readers in accordancewith an exemplary embodiment of the present invention;

FIG. 4 is a schematic diagram of an array of RFID readers in accordancewith a second exemplary embodiment of the present invention;

FIG. 5 is top view of an exemplary mobile RFID reader in accordance withan exemplary embodiment of the present invention;

FIG. 6 is a schematic view an exemplary RFID transceiver in accordancewith an exemplary embodiment of the present invention;

FIG. 7 is a schematic view of an exemplary phase angle estimator inaccordance with an exemplary embodiment of the present invention;

FIGS. 8 and 9 are table views of an exemplary data set in accordancewith an exemplary embodiment of the present invention; and

FIGS. 10 and 11 are graph views of an exemplary data set in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description

Generally, the present invention provides a system and method forlocating objects using Radio Frequency Identification (RFID) tags. Thesystem and method uses the phase of backscatter modulated signals fromthe RFID tags with respect to the fundamental frequency of thetransmitted signals to efficiently and accurately determine the locationof objects.

RFID is a technology that incorporates the use of electromagnetic orelectrostatic coupling in the radio frequency spectrum to identifyobjects to which RFID tags are affixed. RFID systems in general providethe advantage of not requiring direct contact or line-of-sight scanning.A typical RFID system includes an RFID reader and a plurality of RFIDtags that are affixed to the objects of interest. The RFID readerincludes an antenna and also includes or is coupled to a transceiver.The RFID reader uses the antenna and transceiver to transmit radiofrequency signals to the RFID tag. The RFID reader has the ability toselect one or more tags by transmitting an identifier that specifies oneor more of the tags from a plurality of tags. When an RFID readerselects an RFID tag, the RFID tag is put into a responsive mode, withother tags going into a muted state.

When put into responsive mode, the tag transmits data back to the readerby modulating a backscattered signal that is received at the RFID readerantenna. As one example, some tags use variable impedance coupled to theantenna that can be used to change the amount of energy that isreflected back by the tag. These tags can transmit data to the reader byselectively varying the impedance to modulate the backscattered signals.Similarly, these tags can be put into a “muted” or non-responsive stateby selecting and maintaining an impedance value that minimizes thebackscattering from that tag.

Thus, an RFID reader can be used to select one or more RFID tags andretrieve data from the selected tags. As such, RFID tags can be used toidentify and track large numbers or objects. Furthermore, because RFIDtags have a relatively low per-unit cost, they have the ability to tracklarge numbers of objects at relatively low costs.

RFID tags are used in a variety of commercial contexts that require aunique identification system for large numbers of items. As examples,RFID tags are used in everything from department store inventory andcheckout systems to the tracking of military supplies. RFID systems areoften preferred for object identification due to their increased range,lack of a line of sight requirement between a tag and its reader, andhigh multi-tag throughput.

RFID tags are available in a variety of configurations, shapes andsizes. For example, different tags for different applications typicallyhave a shape and size appropriate for its application. RFID tags arecommonly categorized as active, passive or semi-passive. Active RFIDtags include an internal battery used to transmit data and typicallyinclude the ability to read and write greater amounts of stored datathan either passive or semi-passive tags. Passive RFID tags transmit byreflecting and absorbing energy from the RF transmissions from thereader, and use absorbed energy from the reader for data storage,retrieval, and manipulation. Semi-passive tags include an internalbattery that is used for data storage, retrieval, and manipulation,while transmitting data by reflecting and absorbing energy from thereader.

Passive and semi-passive tags are typically lighter and less expensivethan active tags. Passive tags offer a virtually unlimited operationallifetime because they do not require a battery for operation. The tradeoff is that they typically have a shorter read range than active tags,and require a higher output power from the reader. It is important tonote that governmental restrictions in many jurisdictions restrictreader output power to ensure safety and to minimize interferencebetween devices that must share frequency bands.

Passive and semi-passive tags include both read-only tags that areprogrammable once with a unique set of data that cannot be latermodified, and read/writeable tags that can be rewritten one or moretimes using a suitable RFID writing device.

RFID systems can use a variety of techniques to transmit data to andfrom the tag. For transmission to the tag, the data can be transmittedusing any of a variety of modulation techniques, including amplitudemodulation (AM), phase modulation (PM), and frequency modulation (FM).Furthermore, the data transmitted to the tag can be encoded using any ofa variety of techniques, including frequency shift keying (FSK), pulseposition modulation (PPM), pulse duration modulation (PDM) and amplitudeshift keying (ASK).

As discussed above, passive and semi-passive tags transmit byselectively reflecting and absorbing energy from the reader, in aprocess generally referred to as backscatter modulation. Again, inbackscatter modulation, the data can be encoded using a variety oftechniques. For example, the data can be encoded using FSK, where thetag absorb-reflects at one rate to represent a first state (e.g., “one”)and at another rate to represent a second state (e.g., “zero”). Asanother example, the data can be encoded using ASK, where the tagabsorb-reflects at one rate for some duration to represent a first state(e.g., “one”) and ceases back scatter modulation for another duration torepresent a second state (e.g., “zero”). RFID systems also typically usea variety of different frequency ranges, including 30 KHz-500 KHz, 850MHz-950 MHz and 2.4 GHz-2.5 GHz, depending on the regulatory spectrumallocations and performance requirements matched to various applicationrequirements. For example, lower frequency systems typically providebetter electromagnetic penetration through water while higher frequencysystems do not. Lower frequency passive systems commonly operate withina few inches with small reader antennas while higher frequency systemscommonly operate within several feet with similarly sized antennas.Also, lower frequency regulatory allocations are generally morewidespread worldwide and typically allow more output power for passivetags than do higher frequency systems. However, where higher frequencyspectrum is available for RFID, such as within FCC regulated domains,the output power is substantial and allows for robust long-rangeperformance

Turning now to FIG. 1, a schematic view of an object location system 100is illustrated in accordance with an exemplary embodiment of the presentinvention. The object location system 100 includes at least one RFIDreader and a distance calculator. The RFID reader transmits a pluralityof signals to the RFID tag on the object that is to be located. Theplurality of signals transmitted to the RFID tag are selected to havedifferent fundamental frequencies. In response to the transmittedsignals, the RFID tag modulates the plurality of transmitted signals tocreate a plurality of backscatter modulated signals. The RFID readerreceives and demodulates the plurality of backscatter modulated signals.The distance calculator determines the phase in the plurality ofbackscatter modulated signals that were received by the RFID reader. Thedistance calculator determines the phase of the plurality of backscattermodulated signals. From this, the distance calculator determines therate of change of the phase in the backscatter modulated signals versusthe rate of change in the fundamental frequency of the transmittedsignals and uses this information to calculate the distance to the RFIDtag.

In one exemplary embodiment, an array of RFID readers is used todetermine the object location. In this embodiment, each of the RFIDreaders transmits a plurality of signals to the RFID tag, with thesignals from each RFID reader having different fundamental frequencies.In response, the RFID tag backscatter modulates the plurality oftransmitted signals to create a plurality of backscatter modulatedsignals that are received by the array of RFID readers. The distancecalculator determines the phase of the plurality of backscattermodulated signals. From this, the distance calculator determines therate of change of the phase in the backscatter modulated signals withrespect to the rate of change in the fundamental frequency of thetransmitted signals and uses this information to calculate the distanceto the RFID tag. Using the distances to the array of RFID leaders andthe known location of the readers, a highly accurate location can bedetermined using trilateration. Thus, the system and method is able toefficiently determine a highly accurate location for objects thatinclude an RFID tag.

It should be noted that the distance calculator could be implemented invariety of ways. For example, the distance calculator can be fullyimplemented as part of each RFID reader. In another implementation, thedistance calculator can be implemented at a central location, with thephase and fundamental frequency data used to calculate the distancetransmitted to the central computer using any suitable networkingtechnology. Finally, the distance calculator can be implementedpartially in the reader, and partially at a central location. Forexample, the phase determinations can be made at each reader, with thefinal distance and location calculations made at central computer systemlinked to the readers.

As stated above, the plurality of signals transmitted to the RFID tagare selected to have different fundamental frequencies. To have aplurality of signals selected to have different fundamental frequenciesthere need only be at least one transmitted signal with a fundamentalfrequency different from at least one other transmitted signal. Ingeneral additional signals with additional different fundamentalfrequencies provides for increased accuracy of the distance calculation.However, it should be noted that nothing prevents the utilization ofadditional transmitted signals that have the same frequency as othertransmitted signals. Phase measurements taken with transmitted signalshaving the same fundamental frequency as other transmitted signals canbe combined with the other phase measurements to generate a moreaccurate overall calculation of the phase versus frequency rate of thechange.

Turning now to FIG. 2, a method 200 for locating an object isillustrated in accordance with an exemplary embodiment of the presentinvention. In the first step 202, the RFID tag for the desired objectedis addressed. Addressing the RFID tag uses a tag ID to select one RFIDtag among potentially thousands of others. Addressing puts the selectedRFID tag into a responsive mode where it will modulate and backscatterreceived signals from the reader. Tags that are not selected willtypically go into a muted state, where their reflections areintentionally minimized.

The next step 204 is to transmit a plurality of signals from the RFIDreader to the tag, with the signals transmitted having a differentfundamental frequency. Again, to provide a suitable plurality oftransmitted signals there must only be at least one transmitted signalwith a fundamental frequency different from at least one othertransmitted signal, and additional transmitted signals can haveduplicate frequencies. Furthermore, a “fundamental frequency” of asignal, as the term is used in this application, comprises one or moreof the primary frequency components in the signal. For example, thefundamental frequency of a signal can be the frequency of the carriersignal without harmonics. It should be noted that the transmitted signalis not necessarily purely sinusoidal and thus may in fact carryharmonics due to the need for pulse shaping at the receivers.

The plurality of transmitted signals are received by the RFID tag. Inresponse to these signals, the selected RFID tag backscatter modulatesthe plurality of transmitted signals to create a plurality ofbackscatter modulated signals. The RFID reader receives and demodulatesthe plurality of backscatter modulated signals. Because of thefundamental frequency difference in the originally transmitted signal,each of the plurality of backscatter modulated signals received back atthe reader will have a distinct relative phase.

In step 206, the plurality of modulated signals are received back at theRFID reader. The next step 208 is to determine the phase for theplurality of modulated signals. The phase of the received signals can bedetermined in variety of ways, such as channel demodulation. Such amethod will be described in greater detail below.

The next step 210 is to determine the rate of change of the phase withrespect to the rate of change of the fundamental frequency. The rate ofchange of the phase with respect to the rate of change of thefundamental frequency can be calculated from the plurality of phasemeasurements and plurality of transmitted signal fundamental frequenciesusing variety of different techniques. For example, in one application,the rate of change of phase with respect to fundamental frequency isdetermined by performing a linear trend fit of phase measurements andcorresponding fundamental frequency values. The linear trend fit is thenused to generate a more accurate rate of change or “estimated slope” ofphase and frequency.

It should be noted that in some applications steps 204-210 would berecursively continued with more transmissions and phase measurementsuntil the rate of the change could be calculated within a specifiedlevel of accuracy. For example, the steps 204-210 can be continued untilthe linear trend fit generates an estimated slope that is within adesired confidence level, where the confidence level can be calculatedusing any suitable technique such as “goodness of fit” or any othermethod of assessing the variance of the data trend from a straight line.

The next step 212 is to determine the distance between the RFID readerand the RFID tag using the rate of change of the phase of the receivedsignal with respect to the rate of change of the fundamental frequencyof the transmitted signal. As will be explained in greater detail later,the distance (D) between the RFID reader and the RFID tag can becalculated as: $\begin{matrix}{D = {\frac{\Delta\quad\phi}{\Delta\quad f}\frac{c}{4\quad\pi}}} & (1.)\end{matrix}$

Where Δφ is the change in phase of the backscatter modulated signals, Δfis the change in fundamental frequency of the transmitted signals, and cis the speed of light in meters per second or feet per second dependingon the desirable units of the distance measurement. Thus, the distancecan be calculated using the rate of change in the phase (Δφ) of thebackscatter modulated signals versus the rate of change in thefundamental frequency (Δf) of the transmitted signals.

In one application, the method 200 can be applied with an array of RFIDreaders to more completely determine the object location. In thisembodiment, each of the RFID readers performs steps 204 and 206, and thedistance to each RFID reader is calculated in using the phase for thebackscatter modulated signals received at that RFID reader and thefundamental frequencies for the originally transmitted signals. Usingthe distances to at least three RFID readers and the known location ofthose readers, a highly accurate location can be determined usingtrilateration techniques. Thus, the method 200 is able to efficientlydetermine a highly accurate location for objects that include an RFIDtag.

In these multi-reader applications, steps 204 and 206 will typically beperformed on a reader-by-reader basis. For example, the plurality ofsignals will be transmitted and received by a first reader, and aplurality of signals then transmitted and received by a second readerand so on. This provides the advantage of minimizing interferences thatcould result from multiple concurrent transmissions from multiplereaders.

The system and method illustrated in FIGS. 1 and 2 can provide a highaccuracy of object location. For example, an RFID system that uses 900MHz fundamental frequencies can calculate the distance to within atheoretical value of 6 cm when the signal to noise ratio is relativelyhigh. Also, because the system and method can use relatively inexpensivepassive or semi-passive RFID tags, the system and method can be appliedto a high number of objects at a relatively low per-unit cost.

As discussed above, the RFID reader transmits a plurality of signals tothe RFID tag on the object that is to be located, with the signalsselected to have different fundamental frequencies. Again, to implementsignals with a plurality of different frequencies only a least onetransmitted signal with a fundamental frequency different from at leastone other transmitted signal is needed. Furthermore, it should be notedthat that nothing prevents the utilization of additional transmittedsignals that have the same frequency as other transmitted signals. Ingeneral however, transmitting more signals with new frequencies will bedesirable to improve the accuracy of the system.

Several different methods and procedures can be used in selecting thefrequencies of the plurality of transmitted signals. One significantfactor is the regulatory constraints imposed on the system. For example,in some regulatory systems it will be desirable to base frequencyselection on channel availability. In another example, frequencyselection can be randomly selected.

Specifically, in certain bands such as 915 MHz and 2.45 MHz ISM bands,the FCC and other regulatory agencies allow up to 4 watts of transmittedpower if suitable spread spectrum techniques are employed. The objectlocation system can be implemented to randomly select transmissionfrequencies (i.e., channels) within one or more frequency bands and thusmeet the requirements of a spread spectrum system. By randomly selectingfrequencies and otherwise complying with the regulatory framework, theobject location system can thus transmit to the higher power levelallowed such systems. Transmitting at higher power levels increasing theeffective range of the system and thus the overall performance of thesystem.

As another example, other regulatory bodies such as CEPT in Europespecify the use of “listen before transmit” procedures instead of randomhopping used in spread spectrum. In such a system, the transmitterlistens for transmissions at a frequency channel before making its owntransmission in that channel. If the transmitter detects activity inthat channel, the transmitter will switch to the next frequency channel.There it will again listen to detect other transmissions beforetransmitting itself. This process is continued until an availablechannel is found and the transmission is made in that available channel.A variety of different channel selection algorithms are available toimplement such a system based on sensing channel occupation or thetraditional carrier sense multiple access (CSMA) methods. In any event,the object location system can select transmission frequencies using the“listen before transmit” procedure in such a way that it is mostcompatible with the regulatory framework for which the system isdesigned to operate. Without the ability to use such a channel selectionalgorithm the object location system would be limited in the power thatcan be used in transmitting to the RFID tag, resulting in less range andaccuracy for the system.

Turning now to FIG. 3, an exemplary array of RFID readers 300 isillustrated in accordance with an exemplary embodiment of the presentinvention. In this example, the array of RFID readers 300 includes fiveseparate readers. Each reader in the array is used to determine thedistance from the reader to a selected object that includes an RFID tag.Typically, the readers would be spread throughout an area or facilityfor which object location is desired. For example, readers can be spacedin the ceilings or floors of a large warehouse and used to locateobjects within the warehouse.

The distances from at least three of the readers, and the knownpositions of those readers, can then be used to trilaterate anddetermine a highly accurate location of the selected object. Again, thearray of RFID readers transmits a plurality of signals to the RFID tag.Specifically, each RFID reader transmits a plurality of signals havingdifferent fundamental frequencies than at least one signal transmittedby that reader. It should be noted the fundamental frequencies oftransmitted signals from different RFID readers could have the samefrequencies, as they are not typically directly compared.

In response to the signals transmitted by each reader, the RFID tagproduces a plurality of backscatter modulated signals that are receivedby the array of RFID readers. The distance between an RFID reader andthe RFID tag is calculated using the measured phase of the backscattermodulated signals that received by that RFID reader, and thecorresponding fundamental frequencies for the signals originallytransmitted by that reader. Specifically, the change in phase withrespect to the change in the frequency is used with equation 1 describedabove to calculate the difference. Preferably, multiple signals aretransmitted and backscatter modulated to each RFID reader until thechange in phase with respect to the change in fundamental frequency canbe calculated within a specified confidence level. As one example, alinear trend of phase change with respect to fundamental frequencychange can be determined by performing a least squares fit analysis ofthe multiple phase measurements and the corresponding fundamentalfrequencies. This linear trend is a more accurate “estimated slope” ofthe phase change versus the frequency change. As the number of themeasurements increases, the accuracy of the estimated slope and thecalculated distance increases. This process can be continued until theleast squares fit generates an estimated slope that is within a desiredconfidence level, where the confidence level can be calculated using anysuitable technique such as “goodness of fit” or any other method ofassessing the variance of the data trend from a straight line. Thisprocess is then continued until the distance from each reader withinrange of the tag is known at the desired confidence level.

When the distance from the tag to each RFID reader is calculated, thelocation of the object can be determined using these distances, theknown locations of the readers, and a suitable trilateration technique.In the example illustrated in FIG. 3, the distance D1 between the Reader1 and the tag, distance D2 between Reader 2 and the tag, and distance D3between Reader 4 and the tag can be calculated and used to determine thelocation of the object relative to the known locations of the readers.Three distance measurements are generally sufficient to trilaterate theposition of an object, although additional measurements from otherreaders within range of the object can be used to provide additionalaccuracy. Furthermore, as few as one or two readers can be used,although with only one or two readers generating distance measurements,the location of the object cannot be completely disambiguated. It shouldbe emphasized that this is a simplified example of a typical RFID readerarrays, and that the number of readers can be greatly expanded dependingon the size of the area to be covered and the effective range of theRFID readers.

Turning now to FIG. 4, another exemplary embodiment of an RFID readerarray 400 is illustrated. In this embodiment, the array of RFID readersshare one RFID transceiver 406. Specifically, the array of RFID readers400 comprises four physically distributed antennas 402. Instead ofproviding a separate RFID transceiver for each antenna, the array ofRFID readers 400 uses a switch 404 to selectively connect the fourantennas to a single RFID transceiver 406. This approach provides theability to reduce cost in some applications and minimize interferencebetween RFID readers. It should be noted that in other applicationseparate transceivers may be desirable and more cost efficient,depending primarily on the relative cost of the separate transceiversversus cost of separate antennas and cabling.

Again, the RFID reader array 400 can be used to determine the locationof objects that include RFID tags. The transceiver 406 and switch 404would be used to transmit a plurality of signals having differentfundamental frequencies to a selected RFID tag using one or more of theantennas 402. In response to the signals transmitted by each reader, theRFID tag produces a plurality of backscatter modulated signals that arereceived by antennas 402, and selectively passed to the RFID transceiver406 by switch 404. The phase for these signals can then be determined,and the distance between each antenna and the selected RFID tag can becalculated using the rate of change of the phase and the rate of changeof the corresponding fundamental frequencies.

The RFID reader array 400 offers several advantages; the most notablybeing that by sharing transceivers among multiple antennas, the RFIDreader array 400 is able provide the same object location ability at areduced cost and complexity in many applications. For example, animplementation can use multiple antenna sets, each coupled through aswitch to a transceiver, with the multiple transceivers coupled togetherto cost effectively cover a large area with RFID readers.

In systems such as those illustrated in FIGS. 3 and 4, it is desirableto compensate for cable differences and other connection lengths tominimize the effect on the measured phases. This calibration can be doneby either measuring the distance to a known tag location relative to theRFID reader, or by switching the antenna to a known backscatterreference and setting that distance to zero.

In addition to using fixed readers, the system and method can also beapplied to mobile readers. Specifically, a hand-held or other mobilereader can selectively activate RFID tags and determine the distance tothe RFID tag using the methods described above. Additionally, by furtherdetermining the location of the mobile reader, the mobile reader'sdistance measurements can be combined with other measurements from otherreaders to trilaterate the location of the object. Furthermore, themobile unit itself can be used to take multiple distance measurementsfrom different locations that in turn can be used to trilaterate thelocation of the object.

Turning now to FIG. 5, an exemplary mobile RFID reader 500 isillustrated. The mobile RFID reader 500 is exemplary of the type ofmobile readers that can be adapted for use to locate objects using thesystem and method described above. The mobile RFID reader 500 is thusused to determine the location of objects that include an RFID tag. Tofacilitate this, the RFID reader 500 itself includes an RFID tag 502.The RFID tag 502 means that the location of the mobile RFID reader 500can be determined with other RFID readers using the system and methodsdescribed above. With the position of the mobile RFID reader 500determined, the mobile RFID reader 500 can be used as one of an array ofRFID readers to accurately determine the location of an object thatincludes another RFID tag. It should be noted the RFID tag on the mobilereader could be implemented by emulating the behavior of a tag using anantenna, transistor, rectifier and the existing microprocessor on themobile reader. In this example, the transistor is driven by themicroprocessor to create backscatter modulation that imitates aconventional RFID tag.

The mobile RFID reader 500 can communicate with the system using anysuitable protocols and methods. For example, the mobile RFID reader 500can communicate using telephony communication standards, or using otherwireless protocols such as WLAN radios that use IEEE 802.11(b).

It should be noted that while the presence of the RFID tag 502 on themobile reader 500 provides an effective way to determine the location ofthe mobile reader 500, that other methods could also be used todetermine its location. For example, the location of the mobile readercan be determined using the WLAN transmissions to and from the reader.Generally, the trade-off is that location based on phase and frequencydifferences of modulated backscatter signals are much more accurate thanthose based on half-duplex systems such as WLANs that do not produceaccurate phase references.

Turning now to FIG. 6, a RFID transceiver 600 is illustrated inaccordance with an exemplary embodiment of the present invention. TheRFID transceiver 600 is an example of the type of RFID transceiver thatcan be used in RFID reader(s) in the object location system and method.The RFID transceiver 600 is designed to transmit and receive signals toand from a selected RFID tag. Furthermore, the RFID transceiver 600includes a quadrature demodulator. Quadrature demodulators are typicallyused in quadrature amplitude modulation (QAM) systems that combine twoamplitude-modulated signals into a single channel, with two carriers(“I” and “Q”) having the same fundamental frequency but differing inphase, typically by 90 degrees. In quadrature demodulation, the twocarriers are separated, and the data is extracted from each, and thenthe data is combined into the original modulating information. The RFIDtransceiver 600 uses a quadrature demodulator to provide a mechanism fordetermining the phase of the received backscatter modulated signalrelative to the transmitted signal. Specifically, the AC amplitudes ofthe separately demodulated “I” and “Q” channels will be used todetermine the relative phase of the received backscattered signal. Ofcourse, this is just one example, and other transceiver implementationscould be used with other demodulation techniques.

The RFID transceiver 600 includes a modulator 602, a variable gainamplifier 604, a power amplifier 606, a band-pass filter 608, acirculator 610, a band pass filter 614, an automatic gain control 616,demodulators 620 and 622, band pass filters 624 and 626, buffers 628 and630, and phase-locked-loop oscillator 632. The transceiver 600 transmitssignals and receives signals through the antenna 612. Of course,additional antennas could be added using a switch as was described withreference to FIG. 4 above.

In general the transceiver 600 transmits to and receives signals fromselected RFID tags that are in the responsive mode. To transmit data,the transceiver encodes transmission data onto a carrier waveformgenerated by oscillator 632 and broadcasts the signal through theantenna 612 to the RFID tag. Specifically, to transmit data thetransceiver 600 uses the modulator 602 and the variable gain amplifier604 to modulate the carrier signal generated by oscillator 632 with thetransmission data (TX Data). The power amplifier 606 amplifies themodulated signal, which is passed through band pass filter 608. Thecirculator 610 acts as a selective coupler element to antenna 612, wherethe modulated signal is transmitted to the RFID tags, and substantiallyisolated from the directly connected receiver.

To receive data from the tag, the transmitter ceases carrier modulationand the receiver receives the modulated backscattered signal via theantenna, strips the signal from the carrier signal, and converts thestripped signal into an in phase “I” component and a quadrature “Q”component. These components can then be independently digitized and sentto a processor for bit recovery, where they can be interpreted by theRFID reader and/or other related systems. Additionally, these componentscan be used to determine the phase of the received signal relative tothe originally transmitted signal, with the phase of the originallytransmitted signal serving as a reference measurement to determine thechange in phase between the different received signals.

Specifically, the transceiver 600 receives backscatter modulated signalsfrom the RFID tag via antenna 612. The circulator 610 again acts as aselective coupler element, this time coupling the antenna 612 to theband pass filter 614. The received signal may then be amplified by theautomatic gain control 616. This amplified signal may then becarrier-demodulated in quadrature using mixers 620 and 622 and phaseshifter 618, which collectively provide two demodulators. Thisdemodulation results in an in-phase signal I_(AC+DC) and the quadraturesignal Q_(AC+DC). Each of these signals is passed through acorresponding band-pass-filter (624 and 626) and buffers (628 and 630)before the separate signals are further processed.

It should be noted that in this embodiment the demodulator uses the samesignal generated by the phase-lock-loop oscillator 632 that is used forcarrier generation of the originally transmitted signal. As such, thephase of this signal can serve as a reference by which the phase changeof the received signals can be measured. Specifically, by determiningthe phase for multiple received signals with respect to the carriersignal, the relative change in phase between those received signals canbe calculated. Thus, determining the phase difference of the receivedbackscatter modulated signal compared to the originally transmittedsignals provides a mechanism for determining the rate of change in thephase of the plurality of backscatter modulated signals.

Again, this is just one example of an RFID receiver that can be used forobject location. For example, other suitable receivers use separatetransmit and receiver configurations. Yet other suitable receiversreplace the circulator component with a directional coupler. Theadvantage of a directional coupler is much lower cost and smaller sizebut the disadvantage is significant signal loss, hence much lowerperformance

It also should be noted that in many cases the transmissions received bythe antenna would include significant noise and other error components.To minimize such errors it may be desirable to use various errorcancellation techniques. Examples of suitable error cancellationtechniques are found in the patent application entitled “Full-DuplexRadio Frequency Echo Cancellation” by Mark Duron and Raj Bridgelall,filed Oct. 21, 2003, Ser. No. 10/690,390 and assigned to SymbolTechnologies Inc.

With backscattered signals from the RFID tag demodulated, the phase canbe determined and used to calculate the distance to the object. Asdescribed above, the distance calculator determines the phase in theplurality of backscatter-modulated signals that were received by theRFID reader. From the change in phase and the corresponding change infundamental frequency in the originally transmitted signals, thedistance calculator calculates the distance to the RFID tag usingequation 1. The phase differences can then be determined using a varietyof different techniques and devices. As one example, the phase of eachbackscattered signal is referenced to the phase of the originallytransmitted signal.

One method for determining the phase of the received signals is tomeasure the AC amplitude of both I and Q channels and use thosemeasurements to determine the phase angle. That is, the peak-to-peak ACamplitude of the I and Q channel can be averaged over some predeterminedtime period. The relative phase Φ of the received signal as compared tothe carrier phase can be determined as: $\begin{matrix}{\Phi = {\arctan\frac{Q_{amp}}{I_{amp}}}} & (2.)\end{matrix}$

Where Q_(AMP) is the average AC amplitude in the Q channel and I_(AMP)is the average AC amplitude in the I channel. With the relative phase Φof multiple backscatter-modulated signals calculated, the phase changebetween those signals can be calculated and used with the correspondingfundamental frequencies of the transmitted signals to determine thedistance to the tag.

Of course, this is just one example of how the phase of the receivedbackscattered signals can be calculated. Turning now to FIG. 7, anotherexemplary phase angle estimator 700 is illustrated. The phase angleestimator 700 uses the mathematical technique of matrix rotation todetermine the phase of the signals. In the illustrated implementation,the I channel signal I_(AC+DC) quadrature signal Q_(AC+DC) are passed toa DC offset remover 702. This removes the DC portion of the I and Qchannel signals, leaving only the AC portions of each signal.Additionally, noise rejection can be done at this point as well.

The I channel signal I_(AC) an quadrature signal Q_(AC) are then passedto matrix rotation mechanism 704. The AC amplitudes of these signals areloaded into the matrix. Again, these AC amplitudes can be determined byaveraging the AC amplitude over a selected time period. The matrix isthen mathematically rotated until the signal in the Q channel isminimized and the signal in the I channel is maximized. The angle ofmatrix rotation needed to maximize the signal in the I channel is equalto the relative phase of the received signal. In the illustratedexample, the minimization of signal in the Q channel is done using aleast squares estimate minimization technique. Of course, other suitabletechniques could also be used. This method also has the advantage ofmoving all of the signal to the I channel, where the information in thechannel can be recovered and decoded using any suitable technique.Again, with the relative phase of multiple backscattered signalscalculated using the phase angle estimator, the phase change betweenthose signals could be calculated and used to determine the distance tothe tag.

It should be noted that methods and systems described above formeasuring the phase angle of signals cannot always completelydisambiguate the phase of a received signals. Specifically, using thearctangent of the amplitudes will always generate a result of between 0and 2π radians, when in fact the actual phase can be much greater than2π. In general, the original measured phase values are referred to as“wrapped”, and the process of determining the actual, nominal phasevalues from the wrapped values is called “phase unwrapping”.

Thus, phase unwrapping is a technique that can be used to determine thenominal phase change over a linear span of corresponding fundamentalfrequencies. One method of phase unwrapping is to linearize the phaseshift from the wrapped values. Specifically, the phase unwrapping isaccomplished by adding or subtracting multiples of 2π until the phasemeasurement in question shows a consistent trend over a frequency span.

As one example of unwrapping, when a set of monotonically increasingfundamental frequencies are used, a monotonic set of phase measurementsshould result after accounting for any noise. For particular phasemeasurements that do not follow the monotonic trend, they can beunwrapped by adding or subtracting multiples of 2π until they show alinear trend over a linear frequency span. A variety of different phaseunwrapping algorithms are available that can be adapted for this use,such as signal processing tools available in MATLAB.

Turning now to FIG. 8, a table 800 illustrates an exemplary data setfrom which the distance to an RFID tag can be determined using anexemplary embodiment of the present invention. Specifically, the table800 lists 14 transmitted signal fundamental frequencies and acorresponding 14 measured relative phase measurements. It should firstbe noted that this is just one example data set, and that typical datasets could include more or less data points. It should also be notedthat while example data set shows equal distances between fundamentalfrequencies, that this will not be the case in many applications.

In the example of table 800, the frequency order of the transmittedsignals was randomly selected. Again, when random frequency hopping isused the system operates as spread spectrum system and can transmit withincreased power under current regulations. Again, this is just oneexample, and in other cases different frequency hopping procedures canbe used.

The phase measurements illustrated in table 800 are wrapped, againmeaning that the phase measurements are limited to values between zeroand 2π radians. These values thus do not represent the actual relativephase values, and to accurately calculate the distance it is desirableto unwrap the phase measurements. Turning now to FIG. 9, a table 900lists the 14 transmitted signal fundamental frequencies in order offundamental frequency and a corresponding unwrapped 14 measured relativephases. These unwrapped phase values correspond to the actual relativephase of the received backscatter modulated signals. Again, theseunwrapped phase values can be determined by a variety of phaseunwrapping techniques, such as adding multiples of 2π until a consistentlinear phase trend is recovered.

Turning now to FIG. 10, a graph 1000 illustrates the wrapped phasemeasurements and the unwrapped phase measurements of tables 800 and 900.As can be seen, the unwrapping of phase measurements results in phasemeasurements that follow a consistent trend. Using phase unwrappingtechniques, the underlying phase can be determined even in the presenceof significant noise and multi-reflections.

With the unwrapped phase measurements determined, the distance can bedetermined by calculating the rate of change of the phase with respectto the rate of change of the fundamental frequency. As one example, alinear trend fit of the unwrapped phase measurements the fundamentalfrequencies can be performed to determine the rate of change. Turningnow to FIG. 11, a graph 1100 illustrates the unwrapped phasemeasurements of table 900 and graph 1000 along with an exemplary lineartrend calculated from the phase measurements. The linear trend can becalculated from the data using a variety of techniques such as leastsquares fit. When calculated the linear trend gives a more accuratecalculation of the phase change with respect to the frequency change inthe form of the slope of the trend fit line. In the illustrated example,the slope of the linear trend is 9.01E-07 radians/hertz. Whencalculated, the slope of the linear trend fit line can be used as Δφ/Δfin equation 1 to calculate the distance. In this example, using theslope of the linear trend fit line in equation 1 gives a distancemeasurement of 21.4 meters. Thus, the linear fit method is able toovercome noise in the data such as noise created by multi-pathreflections, interference and non-coherent transmissions. Again, this isjust one specific example of how a linear trend fit can be used todetermine the rate of change of the phase and frequency to calculate thedistance to an object with an RFID tag.

The present invention thus system and method for locating objects usingRFID tags. The system and method uses one or more RFID readers and adistance calculator to efficiently and accurately determine the locationof objects that include an RFID tag. The RFID reader transmits aplurality of signals to the RFID tag, with the plurality of signalshaving different frequencies. In response, the RFID tag backscattermodulates the plurality of transmitted signals to create a plurality ofbackscatter modulated signals. The RFID reader receives and demodulatesthe plurality of backscatter modulated signals. The distance calculatordetermines the phase of the plurality of backscatter modulated signalsand determines a rate of change of the phase in the backscattermodulated signals versus the rate of change in the fundamental frequencyof the transmitted signals. The distance calculator then uses the ratesof change information to calculate the distance to the RFID tag.

The embodiments and examples set forth herein were presented in order tobest explain the present invention and its particular application and tothereby enable those skilled in the art to make and use the invention.However, those skilled in the art will recognize that the foregoingdescription and examples have been presented for the purposes ofillustration and example only. The description as set forth is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching without departing from the spirit of the forthcomingclaims.

1. An object location system for locating an object having an RFID tag,the object location system comprising: an RFID reader, the RFID readertransmitting a plurality of transmitted signals to the RFID tag andreceiving a plurality of backscatter modulated signals from the RFIDtag, at least two of the plurality of transmitted signals having adifferent fundamental frequency with a randomly selected frequencydifference; and a distance calculator, the distance calculatordetermining a phase for each of the plurality of backscatter modulatedsignals from the RFID tag, the distance calculator determining adistance to the RFID tag by determining a rate of change of the phase inthe plurality of backscatter modulated signals with respect to rate ofchange in the fundamental frequency of the plurality of transmittedsignals.
 2. The system of claim 1 wherein the RFID reader continues totransmit additional transmitted signals and receive additionalbackscatter modulated signals and wherein the distance calculatorcontinues to determine a phase for each of the additional backscattermodulated signals received and uses the phase for each additionalbackscatter modulated signal to determine the distance from the RFIDreader to the RFID tag is until the distance is computed within aspecified level of accuracy.
 3. The system of claim 1 wherein the rateof change of the phase in the plurality of backscatter modulated signalswith respect to a rate of change in the fundamental frequency of theplurality of transmitted signals is determined by performing a lineartrend fit of the phase in the plurality of backscatter modulated signalsversus the fundamental frequency of the plurality of transmittedsignals.
 4. The system of claim 3 wherein performing a linear trend fitcomprises performing a least squares fit.
 5. The system of claim 1wherein the distance calculator determines the phase difference byunwrapping relative phase difference measurements to result in asubstantially linear phase trend.
 6. The system of claim 1 wherein theRFID reader comprises a mobile reader.
 7. The system of claim 1 whereinthe distance calculator is implemented as part of the RFID reader. 8.The system of claim 1 wherein the distance calculator determines thephase in the plurality of backscatter modulated signals by demodulatingeach of the plurality of backscatter modulated signals into an I channeland a Q channel and measuring an AC amplitude of the I channel and an ACamplitude of the Q channel.
 9. The system of claim 1 wherein thedistance calculator determines the phase in the plurality of backscattermodulated signals by demodulating each of the plurality of backscattermodulated signals into an I channel and a Q channel and utilizing arotation matrix to minimize a portion of signal in the Q channel versusa portion of signal in the I channel.
 10. An object location system forlocating an object having an RFID tag, the object location systemcomprising: an RFID reader, the RFID reader transmitting at least threetransmitted signals to the RFID tag and receiving at least threebackscatter modulated signals from the RFID tag, wherein at least two ofthe at least three transmitted signals have a different fundamentalfrequency; and a distance calculator, the distance calculatordetermining a phase for each of the at least three backscatter modulatedsignals from the RFID tag, the distance calculator determining adistance to the RFID tag by performing a linear trend fit of the phasein the at least three backscatter modulated signals and the fundamentalfrequency of the at least three transmitted signals to determine a rateof change of the phase with respect to a rate of change of thefundamental frequency.
 11. The system of claim 10 wherein the RFIDreader continues to transmit additional transmitted signals and receiveadditional backscatter modulated signals and wherein the distancecalculator continues to determine a phase for each of the additionalbackscatter modulated signals received and uses the phase for eachadditional backscatter modulated signal to determine the distance fromthe RFID reader to the RFID tag is until the distance is computed withina specified level of accuracy.
 12. The system of claim 10 wherein thelinear trend fit of the phase in the at least three backscattermodulated signals and the fundamental frequency of the at least threetransmitted signals is performed using a least squares fit.
 13. Thesystem of claim 10 wherein the at least two of the at least threetransmitted signals have a randomly selected frequency difference. 14.The system of claim 10 wherein the at least two of the at least threetransmitted signals have a frequency selected by selecting a nextavailable frequency channel using a listen before transmit procedure.15. The system of claim 10 wherein the distance calculator determinesthe phase difference by unwrapping relative phase differencemeasurements to result in a substantially linear phase trend.
 16. Thesystem of claim 10 wherein the RFID reader comprises a mobile reader.17. The system of claim 10 wherein the distance calculator isimplemented as part of the RFID reader.
 18. The system of claim 10wherein the distance calculator determines the phase in the at leastthree backscatter modulated signals by demodulating each of the at leastthree backscatter modulated signals into an I channel and a Q channeland measuring an AC amplitude of the I channel and an AC amplitude ofthe Q channel.
 19. The system of claim 10 wherein the distancecalculator determines the phase in the at least three backscattermodulated signals by demodulating each of the at least three backscattermodulated signals into an I channel and a Q channel and utilizing arotation matrix to minimize a portion of signal in the Q channel versusa portion of signal in the I channel.
 20. A method of locating an objecthaving an RFID tag, the method comprising the steps of: =p1 addressingthe RFID tag using an RFID reader, the addressing of the RFID tagputting the RFID tag into a responsive state; transmitting a pluralityof transmitted signals from the RFID reader to the RFID tag, theplurality of transmitted signals having a randomly selected differentfundamental frequency difference; receiving a plurality of backscattermodulated signals from the RFID tag; determining a phase for each of theplurality of backscatter modulated signals; and computing a distance tothe RFID tag by determining a rate of change of the phase in theplurality of backscatter modulated signals with respect to a rate ofchange in the fundamental frequency of the plurality of transmittedsignals.
 21. The method of claim 20 wherein the steps of transmitting aplurality of transmitted signals, receiving a plurality of backscattermodulated signals, and determining a phase in the plurality ofbackscatter modulated signals comprises transmitting, receiving anddetermining a phase for additional transmitted signals and backscattermodulated signals until the step of computing the distance to the RFIDtag determines the distance within a specified level of accuracy. 22.The method of claim 20 wherein the step of computing a distance to theRFID tag by determining a rate of change of the phase in the pluralityof backscatter modulated signals with respect to a rate of change in thefundamental frequency comprises performing a linear trend fit.
 23. Themethod of claim 22 wherein the linear trend fit comprises a leastsquares fit.
 24. The method of claim 20 wherein the step of determininga phase for each of the plurality of backscatter modulated signalscomprises demodulating each of the plurality of backscatter modulatedsignals into an I channel and a Q channel and measuring an AC amplitudeof the I channel and an AC amplitude of the Q channel and by determininga relative phase for each of the plurality of backscatter modulatedsignals by taking an arctangent of the Q channel AC amplitude divided bythe I channel AC amplitude.
 25. The method of claim 20 wherein the stepof determining a phase for each of the plurality of backscattermodulated signals comprises demodulating the received plurality ofbackscatter modulated signals into an I channel and a Q channel andutilizing a rotation matrix to minimize a portion of signal in the Qchannel versus a portion of signal in the I channel.
 26. The method ofclaim 20 wherein the step of determining a phase for each of theplurality of backscatter modulated signals comprises unwrapping relativephase difference measurements to result in a substantially linear phasetrend.
 27. The method of claim 20 wherein the step of transmitting aplurality of transmitted signals from the RFID reader to the RFID tagcomprises transmitting from an array of RFID readers, and wherein thestep of receiving a plurality of backscatter modulated signals from theRFID tag comprises receiving the plurality of backscatter modulatedsignals at the array of RFID readers, and wherein the step of computinga distance to the RFID tag comprises computing a distance from the RFIDtag to each of the array of RFID readers.
 28. The method of claim 27wherein at least one of the array of RFID readers comprises a mobileRFID reader, and wherein the mobile reader includes an RFID tag useddetermine a location of the mobile reader.
 29. The method of claim 20wherein the RFID reader comprises a mobile RFID reader.
 30. The methodof claim 20 wherein the step computing a distance to the RFID tag bydetermining a rate of change of the phase in the plurality ofbackscatter modulated signals with respect to a rate of change in thefundamental frequency of the plurality of transmitted signals comprisesusing at least three determined phases and at least three fundamentalfrequencies to calculate the rate of change.
 31. A method of locating anobject having an RFID tag, the method comprising the steps of:addressing the RFID tag using an RFID reader, the addressing of the RFIDtag putting the RFID tag into a responsive state; transmitting at leastthree transmitted signals from the RFID reader to the RFID tag, whereinat least two of the at least three transmitted signals have a differentfundamental frequency; receiving at least three backscatter modulatedsignals from the RFID tag; determining a phase for each of the at leastthree backscatter modulated signals; and computing a distance to theRFID tag by performing a linear trend fit of the phase in the at leastthree backscatter modulated signals and the fundamental frequency of theat least three transmitted signals to determine a rate of change of thephase with respect to a rate of change of the fundamental frequency. 32.The method of claim 31 wherein the steps of transmitting at least threetransmitted signals, receiving at least three backscatter modulatedsignals, and determining a phase in the at least thee backscattermodulated signals comprises transmitting, receiving and determining aphase for additional transmitted signals and backscatter modulatedsignals until the step of computing the distance to the RFID tagdetermines the distance within a specified level of accuracy.
 33. Themethod of claim 31 wherein the linear trend fit comprises a leastsquares fit.
 34. The method of claim 31 wherein the step of determininga phase for each of the at least three backscatter modulated signalscomprises demodulating each of the at least three backscatter modulatedsignals into an I channel and a Q channel and measuring an AC amplitudeof the I channel and an AC amplitude of the Q channel and by determininga relative phase for each of the at lest three backscatter modulatedsignals by taking an arctangent of the Q channel AC amplitude divided bythe I channel AC amplitude.
 35. The method of claim 31 wherein the stepof determining a phase for each of the at least three backscattermodulated signals comprises demodulating the received at least threebackscatter modulated signals into an I channel and a Q channel andutilizing a rotation matrix to minimize a portion of signal in the Qchannel versus a portion of signal in the I channel.
 36. The method ofclaim 31 wherein the step of determining a phase for each of the atleast three backscatter modulated signals comprises unwrapping relativephase measurements to result in a substantially linear phase trend. 37.The method of claim 31 wherein the step of transmitting at least threetransmitted signals from the RFID reader to the RFID tag comprisestransmitting at least three transmitted signals each RFID reader in anarray of RFID readers, and wherein the step of receiving at least threebackscatter modulated signals from the RFID tag comprises receiving atleast three backscatter modulated signals at each RFID reader in thearray of RFID readers, and wherein the step of computing a distance tothe RFID tag comprises computing a distance from the RFID tag to eachRFID reader in the array of RFID readers.
 38. The method of claim 37wherein at least one of the array of RFID readers comprises a mobileRFID reader, and wherein the mobile reader includes an RFID tag useddetermine a location of the mobile reader.
 39. The method of claim 31wherein the RFID reader comprises a mobile RFID reader.
 40. The methodof claim 31 wherein the at least three transmitted signals have arandomly selected fundamental frequency difference.
 41. The method ofclaim 31 wherein the at least three transmitted signals have a frequencyfrequency selected by selecting a next available frequency channel usinga listen before transmit procedure.
 42. An object location system forlocating objects having RFID tags, the object location systemcomprising: an array of RFID readers distributed around an area, each ofthe array of RFID readers transmitting at least three transmittedsignals to an RFID tag and receiving at least three backscattermodulated signals from the RFID tag, wherein the at least threetransmitted signals from each RFID reader have a fundamental frequencywith a randomly selected fundamental frequency difference; and adistance calculator, the distance calculator determining a phase of theat least three backscatter modulated signals received at each RFIDreader, the distance calculator determining a distance from each RFIDreader by performing a linear trend fit of the phase in the at leastthree backscatter modulated signals and the fundamental frequency of theat least three transmitted signals to determine a rate of change of thephase with respect to a rate of change of the fundamental frequency. 43.The system of claim 42 wherein each of the array of RFID readerscontinues to transmit additional transmitted signals having a fundamentfrequency and continues to receive additional backscatter modulatedsignals and wherein the distance calculator continues to determine aphase for each of the additional backscatter modulated signals receivedand uses the fundamental frequency for each additional transmittedsignal and uses the phase for each additional backscatter modulatedsignal to determine the distance each RFID reader to the tag until thedistance from that RFID reader to the RFID tag is computed within aspecified level of accuracy.
 44. The system of claim 42 wherein thelinear trend fit of the phase in the at least three backscattermodulated signals and the fundamental frequency of the at least threetransmitted signals is performed using a least squares fit.
 45. Thesystem of claim 42 wherein the array of RFID readers includes at leastone mobile reader.
 46. The system of claim 42 wherein the at least onemobile reader includes a mobile reader RFID tag, and wherein other ofthe array of RFID readers are used to determine a location of the mobilereader using the mobile reader RFID tag such that the location of themobile reader can be used to determine a location of other RFID tags.47. The system of claim 42 wherein the array of RFID readers includes aplurality of RFID antennas coupled to an RFID transceiver through aswitch.
 48. The system of claim 42 wherein the distance calculatordetermines the phase in the at least three backscatter modulated signalsby demodulating the at least three backscatter modulated signals into anI channel and a Q channel and measuring an AC amplitude of the I channeland an AC amplitude of the Q channel and by determining a relative phasefor each of the at least three backscatter modulated signals by takingan arctangent of the Q channel AC amplitude divided by the I channel ACamplitude for each of the at least three backscatter modulated signals.49. The system of claim 42 wherein the distance calculator determinesthe phase in the at least three backscatter modulated signals bydemodulating the at least three backscatter modulated signals into an Ichannel and a Q channel and utilizing a rotation matrix to minimize aportion of signal in the Q channel versus a portion of signal in the Ichannel.
 50. The system of claim 42 wherein the distance calculatordetermines the phase in the plurality of backscatter modulated signalsby unwrapping a plurality of relative phase measurements to result in asubstantially linear phase trend.
 51. The system of claim 42 wherein thedistance calculator is implemented as part of the plurality of RFIDreaders.